The present invention relates generally to the detection and quantitative measurement of a selected gas specie. More particularly, this invention relates to said detection and measurement by the technique of selective resonance absorption of an energy frequency band characteristic of the gas specie. In accordance with the invention, an energy beam containing said band is transmitted from a remote source to the situs under test, and is then scattered to the same approximate area as the source. The degree of absorption of energy in said frequency band is measured to determine the concentration of said gas specie. This invention is described herein with specific reference to the detection of methane at a mine working face, but it will be apparent that the invention is limited neither to that gas specie nor to that environment, and may be readily adapted to the detection of other gas species in other environments, provided that the environment provides means for scattering of the energy beam.
It is well known that different gas species have different characteristic resonance absorption spectra, and this property has been used for the identification, detection and quantitative measurement of gases. The prior art discloses that methane can be detected in this way. For example, a Letter by White and Watkins in Applied Optics, vol. 14, no. 12, p. 2812 (Dec. 1975), discloses that an erbium:YAG laser emission centered at 1644.9 nm., overlaps the methane absorption line centered at 1645.1 nm., and suggests that this relation may be used to detect the presence and concentration of methane. The Letter further suggests the possibility of two differential absorption techniques for remote sensing of methane using the erbium:YAG laser--direct transmission measurement, and Mie scattering measurement. For such purpose it is suggested that in the process the laser radiation be tuned on and off the absorption frequency, to distinguish the energy absorption due to methane from attenuation due to other causes.
The use of selective resonance absorption to detect a gas specie, in a system utilizing laser energy and backscattering of the energy, is disclosed in U.S. Pat. No. 2,788,742 to Max Garbuny. Other U.S. patent Nos. disclosing the general technique of selective frequency absorption of laser energy to detect the presence of a particular gas specie, are listed:
3,761,724 James L. Dennis PA0 3,766,380 Robert T. Menzies PA0 3,768,908 Solomon Zaromb PA0 3,820,901 Lloyd B. Kreuzer PA0 3,856,406 Milton L. Noble et al; PA0 4,061,918 Horst Preier et al.
The present invention, as a methane detector, uses an erbium:YAG laser to generate a beam of energy that includes the methane resonance absorption band centered at 1645.1 nm.. The laser transmission is alternated between frequencies on and off said absorption band. One portion of the laser output is transmitted to the situs to be tested, while another portion is directed to a methane reference cell. Because of the environment at the situs to be analyzed, a portion of the laser energy reaching that area is back-scattered or reflected back or scattered toward the laser source.
Detection of the scattered energy is done off-axis of the source, by two spatially displaced detectors. Because of this geometry, the two detectors measure the reflected energy from two spaced volumes, one being referred to hereinafter as the reference volume and the other as the measurement volume. The energy path to and from one volume is greater than the path to and from the other volume. Because of the difference in path lengths, a basis for measuring methane concentration in a remote situs is established. The portion of the laser output directed through the methane reference cell is also detected. This latter measurement provides a reference that enables corrections or compensations in the measurement for small variations in laser frequency in the resonance absorption band.
From the data obtained by detection of the scattered energy from the spaced volumes, and the detection of the energy transmitted through the reference cell, utilizing laser frequencies shifted between on and off the gas specie absorption band, it is possible to compute the relative concentration of the absorbing gas specie, i.e. methane in the illustration being used. Referencing this relative concentration measurement to a predetermined calibration from known methane concentrations, the actual concentration of methane in the measurement volume can be determined.
Although the present invention is not limited to the detection of methane, and is not limited to any particular environment, it is contemplated that it will find an important application in the remote detection of methane at a coal mine working face. A coal mine working face usually has a high concentration of coal dust in the environment, and this provides a rather effective mechanism for scattering the laser energy. Because of the explosion hazard of both coal dust and methane, it is an important safety matter to be able quickly and readily to determine the methane concentration at a mine working face. Likewise, and for that very reason, it is most desirable to be able to make that determination from a remote location, with reasonable accuracy. It is expected that the present invention will fulfill those needs.
It is therefore the object of the present invention to provide for the remote detection and quantitative measurement of the concentration of a particular gas specie, utilizing the resonance absorption characteristics of the gas specie.
It is another object of the present invention to provide for said detection and measurement by irradiating the situs to be measured with energy including energy within said resonsance absorption band, and measuring the energy scattered from said situs.
Another object of the present invention is to provide for said detection and measurement as aforesaid, utilizing a laser as the source of said energy.
Still another object of the invention is to provide for said detection and measurement as aforesaid, wherein said laser frequency is switched between on and off the resonance absorption frequency band of the gas specie.
And still another object of the invention is to provide for said detection and measurement as aforesaid, and detecting the scattered energy at two spaced volumes.
And a further object of the invention is to provide for said detection and measurement, wherein said gas specie is methane.
And a still further object of the invention is to provide for said detection and measurement of methane at a mine working face.
Other objects and various advantages of the invention will become apparent to those skilled in the art, from a consideration of a description of a specific embodiment of the invention presented hereinafter.